Autostart
Warning
This is a fast moving field. This information will be out of date in some detail even before you read it.

OK it needs to autostart and I want it to be easy to start, stop and restart it too.
My procedure to start things up at this point was was: 
  sudo su -s /bin/bash homeassistant
  cd /srv/homeassistant
  source bin/activate
  hass
When I typed this in line by line this just worked. If I ran it as a script I end up back at the /home/pi folder and it didn't start up. Annoyingly the instructions on how to do this are a bad link...
Then I found it at: https://home-assistant.io/docs/autostart/systemd/
which send me off to understand services.

So I made a systemd template: /etc/systemd/system/home-assistant@homeassistant.service
This tells the systemd process to go to folder the /home/homeassistant/.homeassistant and run /srv/homeassistant/bin/hass as a service so it will just run in background and we will control it through the web interface.
  [Unit]
  Description=Home Assistant
  After=network-online.target

  [Service]
  Type=simple
  User=%i
  ExecStart=/srv/homeassistant/bin/hass -c "/home/homeassistant/.homeassistant"

  [Install]
  WantedBy=multi-user.target
You need to reload systemd to make the daemon aware of the new configuration.
  sudo systemctl --system daemon-reload
To have Home Assistant start automatically at boot, enable the service.
  sudo systemctl enable home-assistant@homeassistant

To disable the automatic start, use this command.
  sudo systemctl disable home-assistant@homeassistant
From now on, provided it is enabled, if the pi starts then hass starts.
To manually start Home Assistant now, use the command:
  sudo systemctl start home-assistant@homeassistant
There are 'stop' and 'restart' versions too. The 'restart' is useful when you write a configuration file that blows it all away and you want a second chance to test the edits (or run the restored backup) without rebooting the Pi.

I tested the start, ran the enable and rebooted it.
Its web interface was there within 10 to 15 seconds.

It's obviously tea time again.